In endoprostheses used to replace a joint, a stable connection must be established between the endoprosthesis and the bone structure. In this respect, ceramic materials are less advantageous than other materials, since their dense surface has the effect that connection to bone substance can be established only with difficulty. From WO 99/30634, it is known to apply a titanium alloy to coat the part of an endoprosthesis component that is intended to establish connection to bone substance. The coating has a greater porosity than the ceramic material. The bone substance can grow into the pores and in this way establish a stable connection to the endoprosthesis component.
The titanium alloy does not undergo chemical or metallurgical connection to the ceramic material. The connection between the titanium alloy and the ceramic material is thus a purely mechanical one. According to WO 99/30634, the titanium alloy should be applied to the ceramic material by plasma spraying. For a good mechanical connection, the surface of the ceramic material to be coated should be rough.
Tests have shown that a coating applied in this way does not establish a sufficiently stable connection to every rough surface. If the roughness Ra is below 2 μm, the applied layer lifts from the ceramic material under loading.